1. Field of the Invention
The present invention relates to a semiconductor device in which channel formation regions are formed of a semiconductor film having a crystalline structure. More particularly, the invention relates to a semiconductor device having a circuit made of thin film transistors (hereinafter referred to as TFTs) as well as to a fabrication method for such a semiconductor device. For example, the invention relates to an electrooptical device represented by a liquid crystal display panel as well as to electronic apparatus provided with such an electrooptical device as a component.
Incidentally, the term “semiconductor device” used herein generally denotes devices which function by using semiconductor characteristics, and encompasses semiconductor integrated circuits and electrooptical devices as well as electronic apparatus provided with semiconductor integrated circuits and electrooptical devices.
2. Prior Art
In recent years, the art of fabricating thin film transistors (hereinafter referred to as TFTs) by using a semiconductor film having a crystalline structure with a thickness of about several nm to about several hundred nm (hereinafter referred to as a crystalline semiconductor film) has been developed. The practical application of TFTs to switching elements for use in liquid crystal display devices has proceeded, and it has recently been possible to form semiconductor integrated circuits on a glass substrate by using TFTs.
Silicon is mainly used as the material of a crystalline semiconductor film which is used for TFTs. A silicon film having a crystalline structure (hereinafter referred to as crystalline silicon film) has been fabricated by applying heat treatment or irradiation with laser light (hereinafter referred to as laser treatment) to an amorphous silicon film deposited on a substrate such as glass or quartz by a plasma CVD method or a reduced-pressure CVD method.
For example, in the case of heat treatment, to crystallize the amorphous silicon film, heat treatment needs to be performed at a temperature of 600° C. or more for 10 hours or more. These treatment temperature and treatment time are not necessarily regarded as an appropriate method in terms of productivity of TFTs. Referring to a liquid crystal display device as a product using TFTs by way of example, a larger-sized heat treatment furnace is needed to cope with larger areas of substrates, so that not only does energy consumption increase in a production process, but an uniform crystal is difficult to form over a wide area.
TFTs using a crystalline silicon film fabricated by a conventional art are still inferior in characteristic to MOS transistors using single-crystal silicon substrates. Even if a semiconductor film having a thickness of about several nm to about several hundred nm is crystallized on a different kind of material such as glass or quartz, it is only possible to obtain a polycrystalline structure made of an aggregation of plural crystal grains. In the polycrystalline structure, carriers are trapped by multiple defects present in the crystal grains and grain boundaries, and the performance of TFTs is confined by the trapped carriers.
A representative crystalline semiconductor material to be applied to TFTs is silicon, and a silicon film having a crystalline structure (hereinafter referred to as a crystalline silicon film) is fabricated by crystallizing the amorphous silicon film by applying heat treatment or irradiation with laser light (hereinafter referred to as laser treatment) to an amorphous silicon film deposited on a substrate such as glass or quartz by a plasma CVD method or a reduced-pressure CVD method. However, the semiconductor-film thickness required for TFTs is approximately 10-100 nm, and with this semiconductor-film thickness, it is difficult to form a high-quality crystalline semiconductor film on a substrate made of different kinds of materials such as glass and quartz.
In the case of heat treatment, to crystallize the amorphous silicon film, heat treatment needs to be performed at a temperature of 600° C. or more for 10 hours or more. These treatment temperature and treatment time are not necessarily regarded as an appropriate method in terms of productivity of TFTs. Referring to a liquid crystal display device as a applied product using TFTs, a larger-sized heat treatment furnace is needed to cope with larger areas of substrates, so that not only does energy consumption increase in a production process, but an uniform crystal is difficult to form over a wide area. In the case of laser treatment, an uniform crystal is difficult to obtain, because of the instability of the output of a laser oscillator. Nonuniformity in the quality of crystals causes nonuniformity in the characteristics of TFTs.
As another technique for forming a crystalline silicon film, there is disclosed the art of introducing a metal element which promotes crystallization of silicon, into an amorphous silicon film and fabricating a crystalline silicon film at a heat treatment temperature lower than conventional temperatures. For example, in accordance with Japanese Patent Laid-Open Nos. 7-130652 and 8-78329, a metal element such as nickel is introduced into an amorphous silicon film and a crystalline silicon film is obtained by a 4-hour heat treatment of 550° C.
The crystalline semiconductor film fabricated by any of the above-described related art methods has the property that since crystallization is influenced by a substrate or an undercoat insulating film, plural crystal grains are precipitated and are apt to be oriented with respect to a {111} plane, but the proportion at which the crystal grains are oriented with respect to the orientation plane is low.
A first aspect of the invention is to solve the above-described problem as well as to improve the characteristics of a crystalline semiconductor film obtained by crystallizing an amorphous silicon film and provide a TFT which uses the crystalline semiconductor layer as an active layer.
If an amorphous semiconductor film on a substrate such as glass or quartz is crystallized by either of the above-described methods (Japanese Patent Laid-Open Nos. 7-130652 and 8-78329), a polycrystalline structure is normally obtained. It is considered that the crystallization of the amorphous semiconductor film proceeds on the basis of crystal nuclei which naturally occur at the interface between the amorphous semiconductor film and the substrate. Individual crystal grains in the polycrystalline structure are precipitated with respect to arbitrary crystal planes, but the probability that a crystal is precipitated on a (111) plane where the interfacial energy between the semiconductor film and an underlying silicon oxide is smallest.
If an amorphous silicon film is to be crystallized by introducing an element which promotes crystallization of silicon into the amorphous silicon film, silicides of the element introduced at a temperature lower than the temperature at which natural nuclei occur are formed, and crystal growth based on the silicides occurs. For example, the resultant NiSi2 has no particular orientation, but if the thickness of the amorphous silicon film is made 20-100 nm, NiSi2 is allowed to grow only in a direction parallel to the substrate surface. In this case, since the interfacial energy at which NiSi2 and the (111) plane of crystalline silicon are in contact with each other is smallest, a plane parallel to the surface of the crystalline silicon film is a (110) plane, and crystal grains are oriented in a preferred manner with respect to this (110) lattice plane. If a crystal grows in a columnar shape in a direction parallel to the substrate surface, the degree of freedom exists in the direction of rotation about the columnar crystal, so that the (110) plane is not necessarily oriented and the other planes are also precipitated. The proportion at which the crystal grains are oriented with respect to the (110) plane is not more than 20% of the entirety.
In the case of a low orientation ratio, at a grain boundary where crystals of different orientations meet, it is nearly impossible to retain the continuity of a lattice, so that it can readily be inferred that a large number of unpaired bonds are formed. The unpaired bonds formed at the grain boundary become recombination centers or trapping centers, and lower the transport characteristics of carriers (electrons or holes). As a result, carriers vanish due to recombination or trapped in defects, so that even if TFTs are fabricated with this crystalline semiconductor film, it is impossible to expect TFTs having high field effect mobilities.
In addition, it is nearly impossible to intentionally control the positions of crystal grains, and crystal grains randomly exist, so that the channel formation regions of TFTs cannot be formed with crystal grains having a particular crystal orientation. This fact is considered to be a very serious problem in that the uniformity of the electrical characteristics of TFTs is impaired.